Such earplugs are known as so-called otoplasts which are adapted to an ear canal of a user or which have been formed according to certain predetermined standard shapes, all this in order to provide the sealing between the ear plug and the auditory duct to be as favourable as possible so that “leakage” of sound can be prevented. Hence, in closed form, such an earplug forms a virtually perfect sealing of the auditory duct, so that incoming sound is damped maximally.
The state of the art knows such earplugs, as described in, for instance, U.S. Pat. No. 5,832,094. In the known earplug, a loudspeaker installation is described for connection to a playback device, which loudspeaker is in acoustic communication with the ear canal (see, in particular, FIGS. 3 and 4 and corresponding description of said publication). Further, an acoustic canal is shown that can transmit an ambient sound in a damped manner.
By varying the length of the canal and/or the volume of an acoustic cavity, the damping properties can be determined. The damping shown is not adjustable. International patent publication WO2005041831 shows an acoustic canal with an adjustable acoustic valve. However, in this publication, no additional loudspeaker installation for coupling to a playback device is described.